Author Topic: Briggs 17HP 31 OHV Single Upgrade Path?  (Read 8247 times)

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Offline Scott P.

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Briggs 17HP 31 OHV Single Upgrade Path?
« on: July 10, 2014, 12:13:38 am »
So I am trying to pep up my 31 ci Briggs single. Here is what I have done so far..

-Billet ARC flywheel (currently set full negative which should be stock)
-Governor delete w/ throttle pedal & OMB throttle linkage
-Most recent Briggs OEM head w/ split lock keepers
-EC adjustable needle jet kit
-1.25" exhaust, about 24" long

It currently revs to about 5400 but wont go any higher. Is it safe to assume that the stock valve springs are allowing the valves to float at this point?

What would be next to help this little motor out?

I have already searched a bunch but haven't found any solid info.

I currently have a cheap Chinese tach installed that responds a bit slow, tomorrow I will be installing a genuine Tiny Tach for faster response.

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #1 on: July 10, 2014, 10:29:27 am »
As it sits it doesn't want to rev over about 2300 RPM.

huh>
Thats like idle RPM?

You'll want to balance that one lunger!
Use a degree wheel and get the timing right!
put a cam in it!
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Offline Huffy044

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #2 on: July 10, 2014, 11:17:41 am »
The OHV Briggs can be made to perform very well, however they do have issues:
****Use the crankshaft out of a 12 hp flattie and the oil pan on the 28 through the 31’s, this allows the ARC balance weights to be used. Modification is required to do this to the AVS (see below)****

1.They will not tolerate (break cranks)any timing over 30 deg.-Use a degree wheel to check-do not go by the marks on the flywheel(assuming you get a ARC aluminum wheel)
2. Some 28 and 31 have the coil mounted off to the side-not lined up with the cylinder, this advances the timing-again use the degree wheel.
3. They will break the stock rockers-use EC's aluminum ($200? Or mine for $40, mine are modified stock ones guaranteed for one year)-
4. They WILL pull the rocker studs out of the head-put a plate under the head, use longer studs or use my Stud Bridge ($50.0O also adds rigidity to head to prevent head gasket from blowing)
5.Use a copper head gasket (EC or make your own)
6.AVS engines (identify by oil tube going into block, non AVS goes into oil pan) both 28 and 31 have some that are AVS
7.The AVS WILL break the timing gear, no aftermarket available at this time, (good chance it will also break the oil pan $80 and the cam $150, highly suggest you not build a AVS with a AVS crank in it).
8.You can take a 31 AVS and put a 28 CI flathead oil pan an crankshaft in it IF you weld the top and bottom and the oil fill hole in the block. This will allow you to run the small journal crank, brass syncro eliminators, ARC rod and piston. Block must be machined back down to get the oil pan to seal. I ran two of these engines this year in a Mod and a Pro-had no problems with them, won regional in the mod, was in 2nd at the Nats until a guy jumped the restart and put me on the ground. The deal with building the 31 is you get a factory bore and don’t have to pay $150 to bore a 28 (assuming you cant find a non AVS 31) A 31 CI WILL BEAT A 28 ALL THE TIME, EVERYTIME PERIOD! You need to be able to weld aluminum and access to a milling machine, if you do this make a jig to support the block so the machinist can clamp it on the milling table. I used a heavy peice of square tubing, used one of my broken cranks, machined it down, bolted it to the block through the flywheel side and made plate that bolted to where the head goes on the block. This keeps the block from warping when you weld it. You WILL get porosity when you weld the block, don't worry about it, gasket sealer will seal it.

Want to go fast and the engine live? Do this:

Cut the head .125
Use EC's valves, springs and retainers
Use Precision Cams Cheeta cam, Dyno has one also just as good
Small journal (non AVS) crank, ARC rod, brass eliminator plates.
Copper head gasket
ARC aluminum flywheel
EC's Carb, if you run Pro use the big Tilly (ask George)
No more than 30 deg timing
Roller rockers (EC's or mine)






Stud plate (or my bridge)
Don't try to use dual springs on valves-engine WILL break over 6500 RPM-
Don’t waste your time using a large journal AVS crank, that balances, they WILL break the timing gear.


Note" I no longer make stud bridges or rockers, use EC's
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Offline Scott P.

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #3 on: July 10, 2014, 12:43:28 pm »
Right?! I am going to get a quality tach on it and see what she is doing.

huh>
Thats like idle RPM?

You'll want to balance that one lunger!
Use a degree wheel and get the timing right!
put a cam in it!

THIS IS AWESOME. Thank you. Someone should make this thread a sticky so people don't re-post the same basic questions for the OHV singles.

The OHV Briggs can be made to perform very well, however they do have issues:
****Use the crankshaft out of a 12 hp flattie and the oil pan on the 28 through the 31’s, this allows the ARC balance weights to be used. Modification is required to do this to the AVS (see below)****

1.They will not tolerate (break cranks)any timing over 30 deg.-Use a degree wheel to check-do not go by the marks on the flywheel(assuming you get a ARC aluminum wheel)
2. Some 28 and 31 have the coil mounted off to the side-not lined up with the cylinder, this advances the timing-again use the degree wheel.
3. They will break the stock rockers-use EC's aluminum ($200? Or mine for $40, mine are modified stock ones guaranteed for one year)-
4. They WILL pull the rocker studs out of the head-put a plate under the head, use longer studs or use my Stud Bridge ($50.0O also adds rigidity to head to prevent head gasket from blowing)
5.Use a copper head gasket (EC or make your own)
6.AVS engines (identify by oil tube going into block, non AVS goes into oil pan) both 28 and 31 have some that are AVS
7.The AVS WILL break the timing gear, no aftermarket available at this time, (good chance it will also break the oil pan $80 and the cam $150, highly suggest you not build a AVS with a AVS crank in it).
8.You can take a 31 AVS and put a 28 CI flathead oil pan an crankshaft in it IF you weld the top and bottom and the oil fill hole in the block. This will allow you to run the small journal crank, brass syncro eliminators, ARC rod and piston. Block must be machined back down to get the oil pan to seal. I ran two of these engines this year in a Mod and a Pro-had no problems with them, won regional in the mod, was in 2nd at the Nats until a guy jumped the restart and put me on the ground. The deal with building the 31 is you get a factory bore and don’t have to pay $150 to bore a 28 (assuming you cant find a non AVS 31) A 31 CI WILL BEAT A 28 ALL THE TIME, EVERYTIME PERIOD! You need to be able to weld aluminum and access to a milling machine, if you do this make a jig to support the block so the machinist can clamp it on the milling table. I used a heavy peice of square tubing, used one of my broken cranks, machined it down, bolted it to the block through the flywheel side and made plate that bolted to where the head goes on the block. This keeps the block from warping when you weld it. You WILL get porosity when you weld the block, don't worry about it, gasket sealer will seal it.

Want to go fast and the engine live? Do this:

Cut the head .125
Use EC's valves, springs and retainers
Use Precision Cams Cheeta cam, Dyno has one also just as good
Small journal (non AVS) crank, ARC rod, brass eliminator plates.
Copper head gasket
ARC aluminum flywheel
EC's Carb, if you run Pro use the big Tilly (ask George)
No more than 30 deg timing
Roller rockers (EC's or mine)






Stud plate (or my bridge)
Don't try to use dual springs on valves-engine WILL break over 6500 RPM-
Don’t waste your time using a large journal AVS crank, that balances, they WILL break the timing gear.


Note" I no longer make stud bridges or rockers, use EC's

Offline Bigdaddy73

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #4 on: July 13, 2014, 05:03:07 pm »
Ok, I'm a little confused. Are you welding the pan to the block, or just filling in the old dipstick hole on the block. I ask because I just completely disassembled a 17.5 (model 31) avs and I want it use it to run my first race in september (c\p class). I have the other two motors needed for this judging from what I read. So I guess my questions are ... what am I welding, and how do I keep the timing gear from breaking?

Offline Jeff McKelroy

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #5 on: July 13, 2014, 05:46:58 pm »
Ok, I'm a little confused. Are you welding the pan to the block, or just filling in the old dipstick hole on the block. I ask because I just completely disassembled a 17.5 (model 31) avs and I want it use it to run my first race in september (c\p class). I have the other two motors needed for this judging from what I read. So I guess my questions are ... what am I welding, and how do I keep the timing gear from breaking?


When you are using an AVS engine, the current solution is to either have the AVS sump bushed to use the AVS sump along with a non-avs (35mm main journal) crankshaft, such as the ARC crankshaft which is highly recommended, or a mod28 flathead crankshaft with removable eccentrics to add heavy brass counterweights to the crankshaft, or any other 35mm main journal crankshaft for that matter, but you will have to have the syncro-balance eccentrics machined off of the crankshaft to accept the counterweights. 

The next solution is to use the AVS crankshaft, and the ARC AVS connecting rod, have the eccentrics machined off to accept the brass counterweights.  Your next step will be to contact Rocket Ron for the heavier AVS brass counterweights and steel timing gear.  His contact information is elsewhere in this board.

The next, and apparently common solution is to use the non-AVS sump with an AVS block.  To do this, you simply have to weld up the exposed area around the non-AVS dipstick area, as this protrudes out as an open area and must be filled in to provide proper sealing.  There is also additional information on this board that gives greater detail on how to do this.

I personally would prefer to bush the AVS sump, because I can then correct any crankshaft journal misalignment at this time.  This also allows me to use the ARC Crankshaft, which is a fine piece to use.  And steel timing gears are available from B&S.


Offline Bigdaddy73

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #6 on: July 13, 2014, 09:23:05 pm »
Thanks man. I've a newbie racer on my 1st build. Have an 18 oppy, but it weighs a ton! Got a new 17.5hp for free and loved how lightweight it is so I decided to tear it all the way down so I could re-build with ARC parts...only to find the dreaded AVS on the buick hanging from the crank! Lol. It's a brand new motor, exceptionally clean and lightweight ... I just don't know if it's worth it. I'm only gonna race it for this year. Next year I drop a twin in and race b/p.

Offline shiftyguy

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #7 on: June 16, 2018, 09:29:50 am »
Thanks for all the updr information.   
Just one question though.    I followed your instructions to a tee, however in step 2 you state some have the coil off to the side, anyou say to use the timing wheel,. We've done exactly that , but the hub in the center the bolt holes don't line up anymore, is there a different hub for the offset? Or am I to drill and tap the flywheel?   I'm stumped and it's Saturday.

Offline shiftyguy

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Re: Briggs 17HP 31 OHV Single Upgrade Path?
« Reply #8 on: June 16, 2018, 11:20:06 am »
Thanks for all the updr information.   
Just one question though.    I followed your instructions to a tee, however in step 2 you state some have the coil off to the side, anyou say to use the timing wheel,. We've done exactly that , but the hub in the center the bolt holes don't line up anymore, is there a different hub for the offset? Or am I to drill and tap the flywheel?   I'm stumped and it's Saturday.

 

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